Continued support is sought to study the stereochemistry of important molecular interactions in regulated gene expression. Two classes of specific protein-nucleic interactions will be examined: (1) the interactions of tRNA with its cognate proteins, especially the eukaryotic initiator tRNA whose structure was determined under the aegis of this program, and (2) the interaction of the recently crystallized tryptophan-activated trp repressor with its operator. (1) Interactions of tRNA and cognate proteins: We intend to characterize in atomic detail the specific interactions that stabilize: a) the discriminatory binding of tRNAMet to the E. coli methionine-tRNA synthetase and tRNATyr to E. coli tyrosine-tRNA synthetase, b) the binding of eukaryotic initiator tRNA to initiator-specific proteins such as transformylase, eukaryotic initiation factor 2, and to the preinitiation complexes of the 40S ribosome. These will be approached, where appropriate, by cocrystallization and X-ray crystallography - but the study will depend mainly on photocrosslinking and computerized molecular docking experiments using real-time molecular graphics. (2) Trp repressor/operator: Using X-ray diffraction we intend to solve the crystal structure of the E. coli trp aporepressor in complexes with its corepressor, tryptophan; and with analogues of tryptophan that competitively inhibit its corepressor activity. A major effort will be made to crystallize the ternary complex of trp aporepressor/tryptophan/operator (fragments) in order to visualize directly by X-ray crystallographic analysis the atomic details of a genetic regulatory system.